Tolerance and acclimation capacity of four Swedish urban tree species to heatwaves

Abstract

With progression of global warming and concerning projections of increasing heatwave intensity and frequency, its’ full effects on urban trees are still unclear. Harsh urban conditions like the heat-island effect call for robust tree species that can deal with several stressors. To gain knowledge about which tree species should be used in urban planning for viable future urban greenery, four urban tree species commonly grown in Swedish cities were assessed for heat-tolerance and thermal acclimation abilities to recurrent heatwaves. Namely, Acer platanoides, Betula pendula, Prunus avium and Tilia cordata. It was investigated (1) how heat tolerance varied among the tree species , (2) if they were able to thermally acclimate to heatwaves and (3) if plants repeatedly exposed to heatwaves displayed higher heat tolerance than those that have not. To address this, the plants were either subjected to a heat-treatment of heatwaves, two of those at 33 °C, or a control-treatment with common summer-temperatures. In a third heatwave of 38 °C, the previously control treated plants were exposed to heat as well. Physiological parameters like net photosynthesis, stomatal conductance and water use efficiency were assessed during the heatwaves. Meanwhile, photosynthetic capacity through Vcmax and Jmax as well as dark respiration were assessed post-heatwave. The data shows that the heatwaves of 33 °C had small effects on almost all species, with strongest effects on plant physiology occurring in the last heatwave of 38 °C. Acer platanoides and Tilia cordata showed potential to tolerate heatwaves more than Betula pendula and Prunus avium. However, no signs of thermal acclimation to the heatwaves were found in any species and consequently repeated heatwave exposure did not improve heat tolerance in the stronger last heatwave. This overall suggests that moderate and short heatwaves might be tolerated by the four urban tree species.